Oyster Shell Anatomy

June 11, 2013

… for all you shuckers out there.

Point #1. Oysters have two shells.

Shells are also called valves, hence the whole “bivalve” bit. One of the shells is typically flat, and one is typically curved or cupped. The flat shell is called the “top,” and the cupped shell is called the “bottom.”

So here’s a right side up oyster… top shell is up, bottom shell is down:

Point #2. Oysters have a pointy end and a round end.

The pointy end, or hinge, is the only location where the shells come together.

The hinge is on the left, and the fringe (or “bill”) is on the right:

Some examples of oyster hinges:

On a Blue Pool:

On a Hama Hama Extra Small:

On a Hama Hama Medium:

The hinge is also called the “umbo,” and it’s the oldest part of the oyster. (We use this question frequently in trivia contests, so squirrel that nugget away for later). We’ve heard rumors that you can age an oyster by counting the rings in the umbo. We haven’t confirmed this with shellfish biologists… and some oysters seem to contradict this theory… but it sure sounds good!

Most of the time, tumbled oysters like Blue Pools are easier to shuck than beach grown oysters because they’re more consistently shaped. Sometimes, however, a tumbled oyster’s umbo will start to take things too seriously and will curl over, making it devilishly hard to get the shucking knife in where you want it.

Here’s a semi-freaked out umbo on a Blue Pool (they get way more curled than this):

And a completely calm umbo on a beach grown Hama Hama:

Oysters grow from the bottom shell first, and frequently when you shuck an oyster you’ll find a thin layer of new shell lurking along the mantle. (The mantle is part of the oyster itself, not the shell. We’ll talk about it later.) Go ahead and kick that new shell out when you’re shucking because it’s no fun to eat.

Here’s a picture of new growth coming out of the bottom shell. (The new growth is the pretty, colorful bit.)

Point #3. The adductor muscle runs through the oyster, holding the two shells together.

Once you get the muscle out of the way (either with a knife or with heat) the oyster opens up. This is the whole purpose of shucking, and the reason oysters pop open when placed on a barbecue at medium heat.

When you’re holding the oyster with the cupped side down and the hinge end towards you, the adductor muscle is on the right side of the oyster.

The dark spot on this shell marks where the adductor muscle attached to the bottom shell:

Nerdy fact about the adductor muscle… oysters have “quick” muscles for closing their shells when they sense danger, and “catch” muscles for keeping their shells clamped shut when the tide is out. The quick muscle is crescent shaped, white colored, and located closer to the oyster’s fringe (or bill). The catch muscle is clear and circular and located towards the hinge.

So, to hinge shuck an oyster, insert the knife into the hinge, turn the knife 90 degrees until the hinge pops open, slide the knife along the inside of the top shell to sever the top adductor muscle, remove the top shell, then slide the knife along the bottom shell to sever the bottom adductor muscle. Easy peasy. (Watch a video here and find pictures of this process here.)

Pretty simple, right? But the problem is that not all oysters are as conveniently shaped as our Blue Pools. Sometimes, especially with a wild oyster, it can be difficult to tell top from bottom.

Like, this guy. What the heck is up? Answer: The bottom side.

This oyster is right-side up, which means its bottom (cupped) side is down, but the curve goes the wrong way.

And this oyster is having all sorts of fun:

The only way to get really good at deciphering wild oyster anatomy is to handle lots and lots of oysters. These guys are pretty good at it: